A Boeing 777 jetliner with 307 people on board crashed and caught fire at San Francisco International Airport on Saturday after a flight across the Pacific Ocean from South Korea. Two people were killed and scores injured, authorities said.

The plane, Asiana Airlines Flight 214 from Seoul, apparently hit the lip of the seawall that separates the runway from San Francisco Bay, sources told NBC News, then slammed into the ground and skidded down the runway before coming to rest in an adjacent field, its tail sheared off and the fuselage spewing black smoke. Photos and video from the scene showed passengers sliding down the emergency chutes and walking away.

Despite the deaths and scores of injuries -- many of them serious -- San Francisco Mayor Edwin Lee said at a evening news conference that "This could have been much worse."

Federal investigators said it was too early to determine a cause. A representative of the National Transportation Safety Board arrived on scene late Saturday and took control of the investigation.

The sources who spoke with NBC News said the pilot did not make a distress call before landing. The plane crashed in favorable weather — partly cloudy skies and light wind.

Joanne Hayes-White, the San Francisco fire chief, said there were 307 people on board — 291 passengers and 16 crew -- and all had been accounted for. Authorities said 182 people were taken to one of nine Bay Area hospitals, including 49 with serious injuries. Hospitals reported that the injuries included burns and fractures.

She said first-responders who arrived at the jet saw a handful of survivors emerging from the bay.

“We did observe some of the passengers coming out of the water. But the plane certainly was not in the water. There was a fire on the plane, so the assumption might be that they went near the water’s edge which is very shallow to maybe douse themselves with water.”

The bodies of the two people who were killed were found on the runway, according to Hayes-White.

South Korea's transport ministry said the two who died were Chinese citizens.

Benjamin Levy, who was on the plane, described hearing a lot of screaming after the landing. He told NBC Bay Area that he also saw many head injuries, but that most of the passengers appeared to make it off the plane safely. Fire crews sprayed water and retardant foam to douse the flames.

“We were approaching perfectly well, but we were too low,” he said. “When the pilot realized it, he put some more gas to correct it, but it was too late, so we hit the runway pretty bad, and we started going up in the air again, and we landed pretty hard.”

Stefanie Turner, a witness, told MSNBC that she saw the plane clip the runway with its tail, then come to rest with flames and smoke billowing from the fuselage.

“The tail was too low. Instead of coming in flat it was coming in at, I would say, maybe a 45-degree angle, with the tail far too low,” she said. “It really went through quite a few acrobatics on the runway.”

Besides the 61 Americans, the airline said the jet carried 77 Koreans, 141 Chinese, three passengers from India, one each from Japan and Vietnam and seven whose nationalities were unknown.

The crash — the first involving a jumbo jet in the United States in more than a decade — happened at 11:27 a.m. local time. It left a field of debris down the runway, beginning at the seawall that divides the runway from San Francisco Bay. Pieces of the tail could be seen among the wreckage.

An air traffic control recording captured a controller saying: “Emergency vehicles are responding. We have everyone on the way.”

David Eun, an executive with Samsung Electronics who was on the flight, posted to his Twitter account: “I just crash landed at SFO. Tail ripped off. Most everyone seems fine. I’m ok.” He also posted a photo that showed people walking or running away from the downed plane, including at least a half-dozen who appeared to have slid down an emergency chute.

Eun previously worked at Google and AOL. Sheryl Sandberg, a friend of his and the chief operating officer of Facebook, said that she was supposed to be on Asiana 214 with family and colleagues but that they switched to a different airline to use frequent-flier miles.

The airline said on Twitter that it was investigating and would have news as soon as possible. It offered thoughts and prayers for the passengers and crew.

The deaths are the first in an accident involving a 777, a wide-body, twin-engine jet that has been in service since 1995 and is known as the Triple-7. Before Saturday, the most serious 777 accident was in January 2008, when a British Airways flight landed short and skidded onto the runway, injuring 47 people. The jet in Saturday’s crash had been in service since 2006.

San Francisco International suspended all takeoffs and landings for four hours after the crash and said that some flights were being diverted. Two of its four runways later reopened. The airport advised passengers to check with their airlines.

President Barack Obama, at the Camp David retreat in Maryland, was made aware of the crash and was in touch with federal, state and local authorities, a White House official said.

The last crash of a jumbo jet in the United States was in November 2001, when American Airlines Flight 587, an Airbus A300-600 airliner, crashed in a New York neighborhood. The last fatal crash of any commercial plane in the U.S. was in February 2009, when a Continental Airlines regional flight crashed into a house near Buffalo, N.Y., killing 49 people.

The flight tracking service FlightAware said that Asiana 214 flew about 10 and a half hours after taking off from Incheon airport at 5:04 p.m. local time, about half an hour late. The flight had originated in Shanghai, China.

The service identified the model as a Boeing 777-200, which has a wingspan of almost 200 feet and can carry as many as 440 people. The manufacturing company Pratt & Whitney said the plane was powered by its PW4000 engines.

Federal sources told NBC News that there was no indication of terrorism. The National Transportation Safety Board said it was sending a full team to San Francisco. Boeing said it was gathering information.

“Our thoughts are with everyone affected by today’s incident at SFO,” Boeing said on its Twitter account. “We stand ready to assist the NTSB.”